The present invention generally relates to antennas, and in particular, to a smart reflection antenna system and method for improving the characteristics of a signal.
Antennas are commonly used for relaying information from one point to another. FIG. 1A shows a conventional array-to-array link 81, with an uninformed transmitter. The array-to-array link 81 includes a transmitter 82 coupled to an array of transmitter antennas 21-24. The transmitter antennas 21-24 are collectively referred to as transmitter array 84. Individual transmit antennas or groups of transmit antennas 21-24 transmit independent streams of data. Additionally, the array-to-array link 81 includes a receiver 83 coupled to an array of receiver antennas 27-30. The receiver antennas 27-30 are collectively referred to as receiver array 31. In the embodiment shown by FIG. 1A, there is no communication of information regarding a channel between the transmitter array 84 and the receiver array 31. There is also no modulated backscatter of a carrier signal in the array-to-array link 81. Moreover, the transmitter antennas 21-24 and the receiver antennas 27-30 must be spaced sufficiently far apart so that the independent data streams can be recovered by the receiver array 31.
FIG. 1B shows a conventional array-to-array link 20, with an informed transmitter. The receiver array 31 communicates information about the channel between the transmitter 120 and the receiver 121 to the transmitter array 84. The information enables the transmitter 120 to set the transmitter array weights to achieve a high data rate, using the method of xe2x80x9cwater-fillingxe2x80x9d. However, the communication is typically done as a wireless radio frequency (RF) transmission.
FIG. 2A shows a conventional transponder or a tag 86 that uses passive transmission. An interrogator transmitter (IT) with an antenna 85 generates a carrier signal 87, and there is no power amplifier in the tag 86. Comparatively, in a tag that uses active transmission, a RF carrier signal is generated in a tag, modulated, amplified with a power amplifier, and then delivered to an antenna port for transmission. In FIG. 2A, the carrier signal 87 falls on the tag antenna 88 and is reflected in a time-varying manner by modulating the impedance of the tag antenna 88, using a switch 110 across the terminals 92 and 93 of the tag antenna. The terminals 92 and 93 of the tag antenna 88 are switched between open and closed states, according to an impedance control signal 89, thereby conveying data by modulating the carrier signal 87 and producing a modulated carrier signal 95.
An antenna on an interrogator receiver (IR), the receiver antenna 94, detects the data from the sidebands of the modulated carrier signal 95. This tag transmission technique is called xe2x80x9cmodulated backscatterxe2x80x9d and has the advantage of a dramatic reduction in required power because of the lack of a power amplifier, relative to the active transmitter. This technology is widely used for actively powered Radio Frequency Identification (RFID), or RF tag applications. Its major disadvantage is that the power of the sidebands received at the IR is quite weak, falling off with the square of the distance between the IT antenna 85 and the tag antenna 88, and then falling off again with the square of the distance between the tag antenna 88 and the IR antenna 94. Therefore, assuming that the IT antenna 85 and the IR antenna 94 are close in proximity, the power of the modulated carrier signal 95 decreases with the fourth power of the distance between the tag antenna 88 and the IR antenna 94. In contrast, the power of active transmitters falls off with the square of the distance between the active transmitter and a receiver. A carrier signal that is modulated backscattered also suffers larger fluctuations in multipath fading according to a product of Rician fading factors, instead of a single Rician fading factor for an active transmitter.
FIG. 2B shows that the impedance control signal 89 may be the product of a periodic square wave 90 and an information waveform 91. The information waveform 91 depends on the data stream or information that a tag is transmitting, and is usually a constant if only one bit of information is being transmitted.
Although FIG. 2A depicts an embodiment that modulate backscatters the carrier signal 87, it does not disclose improving the signal-to-noise ratio (SNR) of the modulated carrier signal 95. As for most radio applications in scattering environments, in accordance with prior art, the SNR of the modulated carrier signal may be improved by slight movements of tag antennas. FIG. 3 shows a conventional tag antenna 88 in a tag 86. The tag antenna 88 typically changes its physical characteristics, such as for instance, the position, from position 392 to position 402, to improve the SNR of the modulated carrier signal 95. Alternatively, multiple tag antennas might be specially manufactured or might adapt themselves to change the relative phases and amplitudes of the modulated carrier signal 95 to improve the SNR of the modulated carrier signal 95.
Another conventional embodiment is an uniform linear array (ULA), which is an array of antennas, placed in a straight line and equally spaced in the IT. The array weights are computed to form a beam that is steered to a particular angle. The angle must be known a priori or estimated based on signals previously received by a receiver. This method to calculate the weights of the antennas in the IT is prone to angle estimation error and does not significantly enhance the SNR of the modulated carrier signal. Moreover, the locations of the IT antennas are constrained to be uniformly linear, and therefore cannot be placed in arbitrary locations to provide, for example, macrodiversity.
Hence an unaddressed need exists in the field for a system and method for improving the characteristics of a modulated carrier signal without changing the physical characteristics of the tag antennas such as, for instance, position of the tag antennas; without adopting or manufacturing the tag antennas to produce certain relative RF phases and amplitudes; without restricting the communication of channel information to wireless RF transmissions; and/or without constraints on the locations of antennas in the IT array.
The present invention overcomes the inadequacies and deficiencies of the prior art as discussed herein by providing for a smart antenna reflection system and method that improves the characteristics of a modulated carrier signal.
In this regard, a representative embodiment of a smart reflection antenna system includes an interrogator transmitter (IT) having an IT array of at least two antennas of the IT array. The system also includes a tag having a tag array of at least one antenna. The tag being configured to simultaneously receive the carrier signal from the IT array via the tag array, modulate the carrier signal into a modulated carrier signal; and simultaneously transmit the modulated carrier signal via the tag array. The system also includes an interrogator receiver (IR) having an IR array of at least two antennas, the IR being configured to simultaneously receive the modulated carrier signal from the tag array via multiple antennas of the IR array.
In another embodiment, the invention may be constructed as a method for improving the characteristics of a modulated carrier signal. The method comprising the steps of:
determining a product channel matrix between an interrogator transmitter (IT) antenna array and an interrogator receiver (IR) antenna array, wherein the product channel matrix is the product of a matrix of complex channel gains between the IT antenna array and a tag antenna array and a matrix of complex sideband channel gains between the tag antenna array and the IR antenna array; calculating antenna weight vectors for the IT antenna array and the IR antenna array from the product channel matrix; and controlling the weights of antennas of the IT antenna array with the calculated antenna weight vectors, such that the signal-to-noise ratio (SNR) is maximized for the modulated carrier signal.
Other systems, methods, features, and advantages of the present invention will become more apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present incention, and be protected by the accompanying claims.